The present invention pertains to capacitors having rounded corners. More particularly, the present invention pertains to single-layer, metal-insulator-metal capacitors in which at least one of the metallic layers have rounded corners to reduce the electric fields at the corners, and therefore to reduce the likelihood of breakdown. The present invention further pertains to monolithic microwave integrated circuits (MMICs) including such capacitors.
Capacitors of a metal-insulator-metal structure conventionally have right angle corners. These sharp corners have high electric fields, with the result that breakdown often occurs at the corners. As a consequence, the capacitors must be replaced. This can be a significant problem, particularly in applications such as MMICs which must be reworked or scrapped when a capacitor breaks down. U.S. Pat. Nos. 5,849,624 and 6,049,101 disclose stacked capacitors having rounded upper edges. While such stacked capacitors are useful in many applications, other applications require the use of planar capacitors. U.S. Pat. No. 6,034,864 discloses a multilayer capacitor in which various of the layers have rounded corners. Again, such multilayer capacitors are useful in certain applications, but they are large and bulky, and so are not usable in applications calling for a single layer planar capacitor. Additionally, such multi-layer capacitors are complex to manufacture.
The present invention is a single layer, metal-insulator-metal capacitor having rounded corners on at least one of the metal layers so as to reduce the electric fields and thereby lessen the likelihood of breakdown. By xe2x80x9csingle-layer, metal-insulator-metal capacitorxe2x80x9d is meant a capacitor having a single insulating layer between two metallic layers. Such a capacitor thus includes a first metallic layer with a substantially planar first surface, a substantially planar second surface extending substantially parallel with and substantially coextensive with the first metallic layer first surface, and planar side surfaces joining the first metallic layer first and second surfaces. The capacitor further includes an insulating layer with a substantially planar first surface, a substantially planar second surface extending substantially parallel with and substantially coextensive with the insulating layer first surface and contacting at least a substantial portion of the first metallic layer first surface, and planar side surfaces joining the insulating layer first and second surfaces. Further, the capacitor includes a second metallic layer with a substantially planar first surface, a substantially planar second surface extending substantially parallel with and substantially coextensive with the second metallic layer first surface and contacting at least a substantial portion of the insulating layer first surface, and planar side surfaces joining the second metallic layer first and second surfaces. In accordance with the present invention the second metallic layer first and second surfaces having rounded corners provided by the second metallic layer side surfaces. In one preferred embodiment, the first metallic layer first and second surfaces also have rounded corners provided by the first metallic layer side surfaces.
In a preferred embodiment, the length and width of the insulating layer are less than the length and width, respectively, of the first metallic layer, and the length and width of the second metallic layer are less than the length and width, respectively, of the insulating layer.
Alternatively, the length of the insulating layer can be greater than the length of the first metallic layer, and the width of the insulating layer can be greater than the width of the first metallic layer. In such case, the length of the second metallic layer can be substantially the same as the length of the first metallic layer and the width of the second metallic layer can be substantially the same as the width of the first metallic layer. Alternatively, the length of the second metallic layer can be less than the length of the first metallic layer, and the width of the second metallic layer can be less than the width of the first metallic layer.
In another embodiment, the length and width of the insulating layer can be substantially the same as the length and width, respectively, of the first metallic layer. In a further embodiment, the lengths of the all of the layers can be substantially the same, and the widths of all of the layers can be substantially the same.
The present invention further is a MMIC (Monolithic Microwave Integrated Circuit) including such capacitors.
The present invention additionally is a process of fabricating capacitors of the above type. In one preferred embodiment, the process includes depositing a first photoresist layer onto a wafer, transferring onto the first photoresist layer a pattern of the first metallic layer, developing the pattern of the first metallic layer, depositing metal onto the developed pattern and any remaining photoresist, removing the remaining photoresist and the metal deposited thereon to leave the first metallic layer on the wafer, depositing a second photoresist layer onto the first metallic layer and the wafer, transferring onto the second photoresist layer a pattern of the insulating layer, developing the pattern of the insulating layer, depositing insulating material onto the developed pattern and any remaining photoresist, removing the remaining photoresist and the insulating material deposited thereon to leave the insulating layer on the first metallic layer, depositing a third photoresist layer onto the insulating layer and exposed portions of the first metallic layer and the wafer, transferring onto the third photoresist a pattern having a rounded corners, the pattern being the pattern of the second metallic layer, developing the pattern of the second metallic layer, depositing metal onto the developed pattern of the second metallic layer and any remaining photoresist, and removing the remaining photoresist and metal deposited thereon to leave the second metallic layer with rounded corners on the insulating layer. If desired, the pattern of the first metallic layer can also have rounded corners. In one preferred embodiment, the length and width of the pattern of the insulating layer are less than the length and width, respectively, of the pattern of the first metallic layer, and the length and width of the pattern of the second metallic layer are less than the length and width, respectively, of the pattern of the insulating layer.
In another preferred embodiment, the process of the present invention includes depositing a first photoresist layer onto a wafer, transferring onto the first photoresist layer the pattern of the first metallic layer, developing the pattern of the first metallic layer, depositing metal onto the developed pattern and any remaining photoresist, and removing the remaining photoresist and the metal deposited thereon to leave the first metallic layer on the wafer, depositing an insulating layer onto the first metallic layer and exposed portions of the wafer, depositing a second photoresist layer onto the insulating layer, transferring onto the second photoresist layer a pattern having rounded corners, the pattern being the pattern of the second metallic layer, developing the pattern of the second metallic layer, depositing metal onto the developed pattern of the second metallic layer and any remaining photoresist, and removing the remaining photoresist and the metal deposited thereon to leave the second metallic layer with rounded corners on the insulating layer. Again, if desired, the pattern of the first metallic layer can have rounded corners. Further, the length of the pattern of the second metallic layer can be substantially the same as the length of the first metallic layer, and the width of the second metallic layer can be substantially the same as the width of the first metallic layer.
In another preferred embodiment, the process of the present invention includes depositing a first layer of metal onto a wafer, depositing onto the first layer of metal the pattern of the first metallic layer, etching the first layer of metal to conform with the pattern so as to provide the first metallic layer, depositing an insulating layer onto the first metallic layer and exposed portions of the wafer, depositing a second layer of metal onto the insulating layer, depositing onto the second layer of metal a pattern having rounded corners, the pattern being the pattern of the second metallic layer, and etching the second layer of metal to conform with the pattern so as to provide the second metallic layer. Again, the pattern of the first metallic layer can have rounded corners. Further, the pattern of the first metallic layer can be transferred onto the first layer of metal by depositing a layer of photoresist onto the first layer of metal, transferring the pattern of the first metallic layer onto the photoresist, and developing the pattern. Likewise, the pattern of the second metallic layer can be transferred onto the second layer of metal by depositing a layer of photoresist onto the second layer of metal, transferring the pattern of the second metallic layer onto the photoresist, and developing the pattern.